Fusel oils are a mixture of middle and higher alcohols (fusel alcohols), fatty acid esters, terpenes and furfurals. They form in the course of alcoholic fermentation as by-products of yeast metabolism, and serve as flavor and aroma carriers in beer, wine and spirits. Examples of fusel alcohols are propanols, butanols, pentanols (e.g. isoamyl alcohol) and hexanols. 3-Methyl-1-butanol (an isoamyl alcohol) is the main constituent of fusel oil. 3-Methyl-1-butanol cannot only be obtained from fusel oil, but also, for example, by hydroformylation and reduction of butane isomers.
Guerbet alcohols are specific branched alcohols. They are primary alcohols branched in the beta position to the CH2OH group. Guerbet alcohols are known to those skilled in the art, and some have long been commercially available. They are obtained by what is called the Guerbet reaction, a dimerization reaction which has been known for more than 100 years and can be described by the following formula scheme (R* therein is an aliphatic group):

In the conventional Guerbet reaction, a primary or secondary alcohol is converted to a primary alcohol of about twice the molecular weight, which is alkylated in the beta position to the carbon atom hearing the OH group. For instance, n-butanol is converted to 2-ethyl-hexan-1-ol, hexan-1-ol to 2-butyloctan-1-ol and octan-1-ol to 2-hexyldodecan-1-ol.
The primary or secondary alcohols used for the Guerbet reaction bear at least one hydrogen atom on the carbon atom immediately adjacent to the carbon atom with the OH group; in many cases, they bear two hydrogen atoms, which means that the carbon atom with the OH group is directly adjacent to a methylene group.
The condensation product formed can react further with starting alcohol still present in the reaction mixture, which gives rise to a series of further alcohols with higher molecular weight. The extent to which these side reactions proceed depends in the individual case on the nature of the starting alcohols and the reaction conditions. In addition, it is possible for further side reactions to proceed, which lead to aldehydes, ketones, carboxylic acid or carboxylic esters as by-products. U.S. Pat. No. 3,979,466 states, in this regard (cf. column 1 lines 32-35 therein): “It is further indicated that a plurality of different reactions are likely involved so that the process is highly sensitive and unpredictable as to the effect of particular steps”.
The Guerbet reaction typically proceeds in the presence of a base at elevated temperature with elimination of water and is one way of converting linear alcohols to branched alcohols. Typically, only a single alcohol is used in the Guerbet reaction. However, it is also possible to use two different alcohols; in this case, reference is made to a mixed Guerbet reaction. The first reaction of the type mentioned in history was published by Marcel Guerbet as early as 1899; he had dimerized n-butanol to 2-ethylhexan-1-ol.
Anthony J. O'Lenick states, in Journal of Surfactants and Detergents, Vol. 4 (2001), p. 311-315, that several component steps proceed in the course of the overall reaction, specifically (a) oxidation of the starting alcohol to the aldehyde, (b) aldol condensation, (c) dehydration (water elimination) to give an unsaturated aldehyde and (d) hydrogenation of the allylic aldehyde.
According to O'Lenick, the following information about the component steps is known: (1) The reaction can in principle proceed without catalyst, but is strongly accelerated by the presence of a hydrogen transfer catalyst. (2) At “relatively low” temperatures (130 to 140° C.), the oxidation process, i.e. the intermediate aldehyde formation, is the rate-determining step. (3) At somewhat higher temperatures (160 to 180° C.), the aldol condensation is the rate-determining step. (4) At even higher temperatures, side reactions become dominant.
Since as early as the 1960s and 1970s, the Guerbet reactions for preparation of commercial products have typically been performed using basic catalysts, generally sodium hydroxide or potassium hydroxide. Frequently, in the Guerbet reaction, as well as the base, an additional catalyst is used, in practice usually zinc oxide.
According to U.S. Pat. No. 3,119,880, alkali metal hydroxides, lead acetate and nickel on kieselguhr can be used, in which case nickel serves as a dehydrogenation catalyst.
According to U.S. Pat. No. 3,979,466, alkali catalysts are used in combination with palladium(II) catalysts.
WO 91/04242 describes an improved Guerbet process in which alcohols are dimerized in the presence of a base and of a carbonyl compound at temperatures above 180° C. The starter alcohols used are alcohols having 4 to 22 carbon atoms, preference being given to alcohols having 6 to 18 carbon atoms. The description also mentions, on p. 9, the option of additionally using a cocatalyst, for example complexes or salts of Al, Ni, B, Mg, Cu, Zn, Ti, Zr or a noble metal of group VIII, especially Pt, Pd, Rh, Ir and Ru, without exemplifying this in the examples.
Carlini et al. describe, in Journal of Molecular Catalysis A (2004), p. 65-70, the preparation of 2-ethylhexanol from butanol by Guerbet reaction of bifunctional catalysts based on Cu or Pd and sodium butoxide. For the very specific case of a reaction of methanol with n-propanol to give i-butanol, they first of all mention the use of a Pd/C catalyst in combination with sodium butoxide at temperatures above 200° C. (page 66, left-hand column, at the bottom). On pages 67-69, they then report on their studies of the self-condensation of butanol at 200° C. under Pd(II) and Pd(0) catalysis in combination with sodium butoxide catalysis. The reactions were performed in a 300 ml reactor, using an amount of about 0.5 mol of butanol. Table 1 on p. 68 summarizes the experimental data. Carlini also studied the extent to which the catalyst used remained “stable” under the reaction conditions. He found that there were “solid deposition and leaching effects”, i.e. that the heterogeneous catalyst partly precipitated on the reactor walls and partly went into solution. The leaching of the heterogeneous catalyst used was found to be considerable. Carlini found that 50% of the palladium catalyst used went into solution (cf. p. 69, left-hand column, first paragraph). Carlini arrives at the following conclusion: “This high leaching extent clearly reduces the interest for industrial application perspectives of heterogeneous palladium-based systems”. Carlini's conclusion means that he advises the person skilled in the art, at least for industrial applications, against using palladium-based catalyst systems in Guerbet reactions because, as a result of the leaching, there is an exceptionally high loss of the active substance of the catalyst here.
Matsu-ura et al. describe, in Journal of Organic Chemistry (2006), p. 8306-8308, the Guerbet reaction under iridium catalysis in the presence of alkenes and bases. In the paragraph bridging the two columns of p. 8307, it is stated that, under these conditions, 3-methyl-1-butanol (an isoamyl alcohol) can be dimerized in 50% yield (cf. entry 9 of table 2, p. 8307, right-hand column). The dimerization product has the structure:

An inconvenient and thus disadvantageous feature of the process according to Matsu-ura is the need to work in the presence of an alkene which serves as a hydrogen acceptor. The iridium-containing catalysts used were [IrCl(cod)]2 or [Cp*IrCl2]2.
WO 2009/081727 A1 describes the dimerization of alcohols having a maximum of 4 carbon atoms. The Guerbet reaction here is performed in the presence of complexes of transition metals and a base. The partial hydrogen pressure here is at least 0.1 MPa.